West zone districts witness large-scale movement of butterflies; roadkill ends migration of hundreds midway

The arrival of the Northeast monsoon has kept butterfly observers from Tamil Nadu’s western districts on their toes for the past few days as they were busy observing active movement of butterflies toward the Western Ghats foothills.

Members of The Nature and Butterfly Society (TNBS) observed the movement across Salem, Namakkal, Erode, Tiruppur and Coimbatore districts following the onset of the Northeast monsoon. 

The network of naturalists and members associated with TNBS observed significant movement of milkweed butterflies belonging to the Danainae subfamily, namely Blue Tiger (Tirumala limniace), Dark Blue Tiger (Tirumala septentrionis), Common Crow (Euploea core) and Double-branded Crow (Euploea sylvester). Field observations recorded a steady movement of these butterflies toward the west and southwest. According to TNBS, which has been monitoring the phenomenon across the region since 2013, the current migration mirrors its observations of a movement in mid-September after a few spells of rain.

Among significant observations recorded by TNBS, Blue Tigers passed Rasipuram in Salem at a rate of 72 individuals per minute. According to TNBS, the movement peaked from October 17 (from 8.30 a.m. to 4 p.m.) across Coimbatore district, spanning from Mettupalayam to Singanallur with movement rates soaring from five to 135 butterflies per minute. It continued on October 18 across Erode and Tiruppur, with ongoing sightings of migrating Crows and Tigers. Reasonable rain was noted across Tamil Nadu, particularly in the regions involved in the migration corridor during these movements. 

An observation that raised concerns among naturalists was that butterflies that typically migrate at heights between five to 25 meters and above were widely observed moving much closer to the ground level. The low-level flight path resulted in the roadkill of butterflies in hundreds across the region. Butterflies that reached the Western Ghats foothills actively swarmed nectar plants available in the regions.

The current migration after the onset of the monsoon matches the pattern that had been observed by TNBS since 2013 -- west/southwest movement. Another migration from the Western Ghats hills towards the Eastern Ghats and plains normally occurs during April and May in the following year, coinciding with the arrival of the Southwest monsoon. 

Along with observers from TNBS from Salem, Erode, Tiruppur and Coimbatore, members of the Coimbatore Nature Society were also engaged in field monitoring. TNBS is a part of the wider ‘Butterfly Migration India’ watch group.